Nancy Davis may have been attacked in Mexico for her truck, police say.

Reporting from Denver and Mexico City — A 59-year-old American missionary was shot in the head and killed in northern Mexico, possibly because one of the local drug cartels coveted her heavy-duty pickup truck, authorities said Thursday.

Nancy Davis' husband, Sam, drove the bullet-riddled blue 2008 Chevrolet against traffic to the border Wednesday afternoon. He crossed the bridge into Pharr, Texas, where he told authorities that the couple had been ambushed about 70 miles south of the border on a Mexican highway by gunmen in a black pickup, according to the Pharr Police Department.

Davis was rushed to a hospital in McAllen, where she died. Friends told reporters she was a longtime missionary with vast experience in the increasingly dangerous area of northern Mexico, which has been racked by drug violence for several years. Local police said the type of truck the Davises drove was prized by Mexican cartels.

The shooting was reported to have taken place near the town of San Fernando, in Tamaulipas state. San Fernando was the site in August where 72 immigrants, mostly from Central America, were abducted and slain in the single largest massacre of Mexico's raging drug war.

Gunmen from the notorious Zeta cartel were suspected in the migrant massacre. They control much of Tamaulipas and are locked in a vicious battle with the rival Gulf cartel for supremacy.

The Davises had spent decades as missionaries in Mexico and owned a home in the nearby state of Nuevo Leon, friends told reporters. They had also founded a group called the Gospel Proclaimers Missionary Assn. in Weslaco, Texas.

The God's Missionary Church in Beavertown, Pa., alerted its congregation about the attack via Twitter. "Long time missionary Nancy Davis has gone to Heaven," the church said. "Serving in that country for over 35 years with her family, she has now given her very life for her people."

American authorities said the investigation was largely in Mexican hands. "Mexico being a sovereign nation, we ask the involved entities over there to aggressively pursue cases such as this," said Erik Vasys, an FBI spokesman.

The Mexican federal government condemned the shooting in a statement Wednesday evening, and the Tamaulipas state government did as well Thursday, while also pledging to cooperate with authorities investigating the killing.

Few crimes are ever resolved in Mexico, however, least of all in a violent state such as Tamaulipas, where cartels hold massive sway.

The attack revived concerns about violence from Mexico's drug wars spilling across the border. Crime has either dropped or held steady along most of the U.S. side of the border, which includes some of the safest parts of the country, according to FBI crime statistics. But there have been signs in recent months that the calm may not hold.

Katherine Cesinger, a spokeswoman for Texas Gov. Rick Perry, said the killing was a reminder of the need for greater border protection. "The federal government has not done nearly enough to secure the border," she said.